Bimodal particle size distribution

As the research on coal/water slurries shows, the suspension viscosity can also be reduced by close control of the particle size distribution bimode particle distributions seem to be the most beneficial. 

Particle size distribution is usually plotted on a log-probabiHty scale, which allows for quick evaluation of statistical parameters. Many naturally occurring and synthetic powders foUow a normal distribution, which gives a straight line when the log of the diameter is plotted against the percent occurrence. However, bimodal or other nonnormal distributions are also encountered in practice. 

The significance of this novel attempt lies in the inclusion of both the additional particle co-ordinate and in a mechanism of particle disruption by primary particle attrition in the population balance. This formulation permits prediction of secondary particle characteristics, e.g. specific surface area expressed as surface area per unit volume or mass of crystal solid (i.e. m /m or m /kg). It can also account for the formation of bimodal particle size distributions, as are observed in many precipitation processes, for which special forms of size-dependent aggregation kernels have been proposed previously. 

The first process in all cases is the production of the oxidiser in a suitable fine crystal size. A bimodal particle size distribution, obtained by mixing very fine with slightly coarser particles, often gives the best product. The fuel/binder is frequently prepared as a prepolymer so as to assist mixing and also to reduce the time of the later curing process. 

Example 15-1 Determine the pressure gradient (in psi/ft) required to transport a slurry at 300 gpm through a 4 in. sch 40 pipeline. The slurry contains 50% (by weight) solids (SG = 2.5) in water. The slurry contains a bimodal particle size distribution, with half the particles below 100 pm and the other half about 2000 gm. The suspension of fines is stable and constitutes a pseudohomogeneous non-Newtonian vehicle in which the larger particles are suspended. The vehicle can be described as a Bingham plastic with a limiting viscosity of 30 cP and a yield stress of 55 dyn/cm2. 

In a similar study, Allen and co-workers (1996) determined the particle size distribution for 15 PAHs with molecular weights ranging from 178 (e.g., phenan-threne) to 300 (coronene) and associated with urban aerosols in Boston, Massachusetts. As for BaP in the winter (Venkataraman and Friedlander, 1994b), PAHs with MW >228 were primarily present in the fine aerosol fraction (Dp < 2 /Am). A study of 6-ring, MW 302 PAH at the same site showed bimodal distributions, with most of the mass in the 0.3- to 1.0-/zm particle size size range a smaller fraction was in the ultrafine mode particles (0.09-0.14 /xm) (Allen et al., 1998). For PAHs with MW 178—202, the compounds were approximately evenly distributed between the fine and coarse (D > 2 /am) fractions. Polycyclic aromatic hydrocarbons in size-segregated aerosols col-... 

Figure 12.28 shows the particle surface area size distribution before the Mount Pinatubo eruption (Fig. 12.28a), inside the main aerosol layer several months after the eruption (Fig. 12.28b), and almost two years after the eruption (Fig. 12.28c). (See Chapter 9.A.2 for a description of how particle size distributions are normally characterized.) Prior to the eruption, the surface area distribution is unimodal, with typical radii of 0.05-0.09 /xrn and a number concentration of l-20 particles cm 1. In the main stratospheric aerosol layer formed after the eruption, the distribution is bimodal...

PBX-9404-03 HMX 94, NC 3 Tris(j8- chloroethyl) - phosphate 3%. The OS denotes a bimodal particle size distribution of HMX... 

The particle size distribution suggests that nonirradiated PTFE powder has a broad particle size distribution compared to 500 kGy-irradiated PTFE powder. The nonirradiated PTFE powder shows a characteristic bimodal distribution compared to the unimodal distribution of 500 kGy-irradiated PTFE powder (Fig. 14). This specific bimodal distribution clearly signifies that nonirradiated PTFE powder is mainly composed of bigger agglomerates that tend to reagglomerate. By contrast,... 

ABS compositions with bimodal particle size distributions of the grafted rubber can be prepared by emulsion graft polymerization techniques. The preparation of ABS types by emulsion polymerization consists in brief of (13) ... 

In some cases, due to coalescence mechanisms (Oswald ripening), bimodal particle-size distributions are observed. 

It has been assessed that bimodal particle size distributions consisting of a population of small and large particles may exhibit a better toughening effect than unimodal ones, due to a synergistic effect (Chen and Yan,... 

Particle size distribution of a seeded PVC latex (bimodal distribution)... 

HRTEM was used to determine the final metal particle size. A representative photograph is shown in Figure 5. Multiple photographs were analyzed and the size of at least 230 particles was measured to determine the average particle size and the particle size distribution (Figure 6, grey). The particle size distribution appears to be bimodal, with maxima at 14 and 22 A. The average particle size (as determined with HRTEM) for this sample is 18 A. 

In conclusion, the direct reduction is a fast process taking place between 150 and 200°C, resulting in relatively large particles with an average size of 14-16A and a bimodal particle size distribution. The desorption of NH3 and H2O at 211°C is caused by the decomposition of NH4NO3 in an independent process of the reduction. 

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